HTML5 is not backwards compatible... it redefines some elements with incompatible semantics.... a UA must treat documents in a way that is guaranteed to differ both from how a HTML4 (or earlier) UA would treat it, and from how the author originally intended it to be treated.
Can you give me an example of an actual RW problem this will cause? HTML offers such limited presentational semantics I can't see how minor changes to an element's "meaning" will affect anyone(?)
So HTML5 can be adopted gradually/progressively as browser support for individual features is added (like XMLHttpRequest has been adopted), but XHTML2 required a year-zero, all-or-nothing approach to adoption.
Actually, this isn't entirely true. Exluding the interactive (forms and events) stuff, XHTML2 can be rendered in any current browser that supports XML + CSS2... you can easily build a DTD ...take the "classic" forms module from XHTML1.1...
Yes, after you've re-written all your content in XHTML2, it's possible to make it work again in current browsers, like it did before. I think this re-writing qualifies as an all-or-nothing, year-zero approach ;)
HTML5 *doesn't* require us to rewrite entire sites and publishing systems to use a new feature. So when for example Canvas is widely supported we'll be able to add it to an existing website (HTML4/XHTML1/tag-soup) and it'll just work. Probably. :)